The present invention relates to a thermal ink jet printhead and method of manufacture thereof, and more particularly to a thermal ink jet printhead having an improved flat, top surface heater plate that is bonded to a channel plate during fabrication.
Typically, thermal ink jet printing systems each include an ink jet printhead for ejecting ink droplets on demand by the selective application of current pulses to an array of thermal energy generators. The thermal energy generators are located individually in parallel, capillary-filled ink channels in the printhead. Each thermal energy generator, usually a resistor, is located as such at a predetermined distance upstream of the droplet ejecting nozzle or orifice of the channel. U.S. Re. 32,572 to Hawkins et al exemplifies such a thermal ink jet printhead and several fabricating processes therefor.
Conventionally, each such printhead is composed of two separately fabricated parts that are aligned and bonded together. One such part is a substantially flat substrate or plate (the heater plate) which contains a linear array of heating elements and related addressing elements. The other part is also a substrate or plate (the channel plate) having at least one recess anisotropically etched therein to serve as an ink supply manifold when the two parts are bonded together. Additionally, other recesses or grooves forming a parallel array are also etched in the channel plate and form ink channels upon the bonding of the plates. The grooves are each formed such that one end thereof communicates with the ink supply manifold, and the other end is open so as to function as an ink droplet ejecting nozzle of the resulting channel.
As described for example in the Hawkins et al patent, many printheads of this type can be manufactured simultaneously. To do so, a plurality of sets of heating element arrays with their addressing elements are fabricated on a first silicon wafer to form the heater plate, and alignment marks are placed thereon at predetermined locations. A corresponding plurality of sets of channel grooves and associated manifolds are then formed in a second silicon wafer to constitute the channel plate, and alignment openings are etched in the channel plate at predetermined locations. The heater and channel plates are then aligned via the alignment openings and alignment marks, bonded together, and diced into many separate individual printheads.
Improvements to such two part thermal ink jet printheads are described for example in U.S. Pat. No. 4,638,337 to Torpey et al which discloses a printhead similar to that of Hawkins et al, but has each of its heating elements located in a recess or heater pit. The walls of the recess or heater pit function to prevent lateral movement of heated ink moving over the heater element towards the nozzle. As such, the recess or heater pit acts to prevent the sudden release of vaporized ink to the atmosphere, an occurrence known as "blow-out". "Blow-outs" as such are undesirable because they can cause ingestion of air into the printhead and hence interruption of the printhead operation. In the Torpey et al patent, a thick film insulating layer of an organic structure, such as polyimide, Riston.RTM. or Vacrel.RTM., is formed on top of the heater plate prior to bonding. The recesses or heater pits are formed in this thick film layer. As a result of this improvement, the top surface of the heater plate for bonding to the channel plate is therefore that of a thick film insulating layer. The thick film insulating layer as such serves as an ink insulation, and as a protection layer for the heating and circuit elements of the heater plate. As such, the thick film layer is preferably made of polyimide because polyimide is impervious to water--a major common component of inks used in ink jet printheads.
In the manufacture of the two plate-printhead as above, the top surface of the heater plate as such must be precisely and thoroughly bonded to the channel plate in order to effectively isolate ink within the channels. Typically, a thin uniform layer of adhesive material is used for such bonding. The flatness of the bonding surfaces of the plates, and the thickness of the adhesive layer are critical to the effectiveness of such bonding. The thickness of the adhesive layer, for example, should not be insufficient, nor should it be too much. Too much or too thick an adhesive layer tends to cause the adhesive to spread or wick from the coated surface into adjacent channels, thereby interfering with consistent printhead firing characteristics. On the other hand, insufficient adhesive layer thickness, for example, leads to poor adhesion or poor bonding between the heater and channel plates, and hence to a host of problems including, cross-talk, poor channel firing consistency, and ink droplet size variations.
Such poor adhesion with its attendant problems can also result when the bonding surfaces of the plates are not sufficiently flat. The degree of flatness or non-flatness of the bonding surfaces of these plates, particularly that of the heater plate, is significantly determined in part by the materials forming the plate, and by its process of fabrication.
Conventionally, the channel and heater plates are each fabricated from a silicon wafer. In the case of the channel plate, the recesses or grooves therein are formed in the wafer, for example, by an anisotropic etching process. In the case of the heater plate, patterned layers of heating elements and their related addressing circuit elements are fabricated on the silicon layer along with protective and insulative layers including the top, thick polyimide insulation layer.
Unfortunately, however, the polyimide material which form the top bonding surface has a tendency to produce unwanted surface topographical variations. Such unwanted surface variations are caused by formations such as raised edges or "lips" (1-3 microns high) which occur around any photoimaged edge. For example, such formations occur around the edges of the heater and bypass, pits. Such raised edges and "lips" formations ordinarily affect the flatness of the top bonding surface of the heater plate, and thus tend to result in undesirably poor adhesion or poor bonding between heater and channel plates of printheads. Another undesirable type of polyimide top surface topographic formation occurs as "edge beads" or raised areas at the edges of the heater plate. The edge bead on a 4 inch diameter heater plate, for example, can be on the order of 0.5 inch wide extending radially from the outer edge thereof, and can have a thickness several micrometers higher than the rest of the polyimide layer.
In addition to the above mentioned and unwanted polyimide top surface formations, it has also been found that poor adhesion and poor bonding can result between the heater and channel plates as a result of area to area variations in the overall thickness of a completely fabricated heater plate. Such area to area variations which manifest themselves as high and low areas in the top surface of the top polyimide layer are caused, for example, by the existence of nonuniform, thin film sublayer patterns in the heater plate. Examples of such non-uniform sublayer patterns are thin film active layers of heating elements and integrated circuit elements. Such active layer elements are formed, for example, by photopatterning a uniform layer of active thin film material and then etching off active thin film material from the non-circuit areas of the sublayer. The remaining sublayer patterns are believed to cause corresponding patterns of high areas on the top (polyimide) surface of the heater plate.